In a measurement while drilling system the downhole equipment requires electrical power which is provided by a power supply that includes a motive power source to extract mechanical energy from kinetic energy of drilling fluid passing through the drill string, and an electrical power source in the form of a generator or an alternator coupled with the motive power source. In the operation of such a power supply it it important to regulate the speed of the generator or alternator to operate within its design limits so that the downhole electrical equipment will be provided with a relatively constant and adequate supply of electrical energy for proper operation. Regulating the operation of this power supply requires the consideration of several factors including electrical loading of the electrical power source due to operating demands of the associated electrical system; pressure and flow rate variations in the drilling fluid flow, and occluding, plugging or clogging of the motive power source with particulate matter that is carried in the drilling fluid.
The present invention concerns the flow of drilling fluid through the motive power source portion of this apparatus. Typically the motive power source includes a turbine with its blade or rotary element mounted on or operably connected to the rotatable shaft of an alternator. The turbine receives high pressure drilling fluid at its inlet and discharges the fluid at a lower pressure at its outlet. The turbine is designed so that motion of a valve member relative to the inlet will regulate the quantity of drilling fluid passing through the turbine in relation to the quantity of drilling fluid by-passing the turbine's inlet.
In prior constructions when the inlet to the turbine is stationary in relation to its location in the drilling fluid flow the turbine must accept a theoretically fixed proportion of the drilling fluid. This construction is undesirable due to fluctuations in the drilling fluid flow rate and pressure which also vary the fluid flow rate through the turbine and thereby vary its operating speed. This variation will directly effect the alternators performance. Also this arrangement does not account for clogging or plugging of the turbine by particulate material and the like that is carried through the drilling fluid. Because the power output from the alternator can obviously vary if the turbine is occluded or plugged and made inoperable, this prior construction is not desirable.
In another prior construction of this equipment a movable valve member can be provided which is spring urged to a position that directs substantially all of the drilling fluid to pass through the turbine and relaxed from this position only in response to the drilling fluid pressure acting in opposition to the spring. This arrangement while providing some degree of regulation for fluid flow through the turbine is not responsive to rapid changes of the differential across the turbine. Also, it is not responsive to short duration pressure pulses in the mud flow that tend to change the speed of the turbine. Through experimentation it has been found that the pressure differential across such a turbine is important to regulating the rotational speed thereof and that the two above described prior art constructions are inadequate to provide consistent regulation of the power supply.